Cellulose nanofiber/MXene/AgNWs composite nanopaper with mechanical robustness for high-performance humidity sensor and smart actuator

Two-dimensional (2D) MXene provides large surface area for the adsorption and interaction of H2O molecules, which is highly desirable for the construction of humidity-sensing materials. However, the drawbacks, involving easy stacking and poor mechanical strength, pose a huge challenge to its practical application. Herein, inspired by nacred "brick-mortar" nanostructure, a nanopaper humidity sensor made of flexible TEMPO-oxidated cellulose nanofibers/MXene/silver nanowires (TOCNFs/MXene/AgNWs) was developed by vacuum-assisted filtration self-assembly strategy. The synergism of 1D TOCNFs, AgNWs "mortar" and 2D MXene "bricks" endows the nanopaper with superb tensile strength (146.3 MPa), modulus (16.9 GPa), and superior bending durability, and the obtained TOCNFs/MXene/AgNWs nanopaper humidity sensor exhibits high response value of 90% at 97% RH with a low MXene addition of 20 wt%. Furthermore, the unique photothermal response property of MXene accelerates the desorption of H2O molecules during sensor recovery, realizing the reversible sensing performance. The proposed humidity sensing mechanism lies in the variation of MXene interlayer d-spacing induced by adsorbing ambient H2O molecules and the swelling of TOCNFs. Lastly, the sensor demonstrates the possibilities in human respiration monitoring, non-contact sensing, and humidity actuating.